This invention relates to the art of pipelines, and specifically, pipelines which are buried in underground trenches along their length.
Pipelines are used to transport fluids, typically oil, gas and other petroleum products, across long distances. In the current art of installing new pipelines, the pipe is lowered into trench and laid on top of the support benches, which are spaced along the length of the trench floor. The trench containing the newly laid pipe is then backfilled normally in two stages. A layer of uniform, rock-free subsoil is introduced into the trench so that it flows around and beneath the pipe between the spaced support benches, fills up the open area between the sides of the pipe and the trench walls, and covers the pipe to a specified height over the pipe. In the art this procedure is termed xe2x80x9cpipeline paddingxe2x80x9d or xe2x80x9cpaddingxe2x80x9d. The second backfill stage is to then utilize the remaining spoil previously excavated from the trench to complete the trench backfill. After the backfilling is completed, a hydrostatic test is conducted. The pipe is filled with water and placed under high pressure for a period of time, usually 24 hours, in which the pressure is monitored and the pipe section is tested for leaks.
After the pipeline has been installed and hydrostatically tested an additional test is conducted. Devices called xe2x80x9csizing pigsxe2x80x9d are sent through the pipe. It is during this procedure that damage to the pipe is found and identified. All dents and buckles as well as most xe2x80x9cout-of-roundxe2x80x9d sections must be dug up and repaired before the pipeline can be placed into service. This repair work is very expensive and time consuming. The support benches hold the pipeline above the trench floor both during backfilling and while the pipe is in service. Because rocks on the trench floor may dent the pipe or damage the protective coating on the outside of the pipe, it is very important that the pipe does not come into contact with the trench floor during service as well as during installation. Sandbags are commonly used as support benches. These bags are can be stacked to any desired height. Another popular bench material is high density foam blocks. These blocks are strong enough to support the pipeline without collapsing. U.S. Pat. Nos. 4,068,488 to. Ball discloses the use of inflatable support pads that temporarily support the pipeline during backfilling. These bags are removed as the backfill material approaches the bag during backfilling. The patent calls for a granular bedding/padding material to be forcibly injected under the pipe to obtain full compaction. The bedding, being fully compacted prior to the removal of the temporary pad, will then support the pipe at the same pipe position. U.S. Pat. Nos. 4,488,836 and 4,806,049 to Cour disclose the use of bags filled water to install pipeline in ocean bottoms or trenches that are not stable. The purpose of which is to keep the trench walls from collapsing before the pipe can be installed. The pipe is then laid on top of the pressurized bladder which has filled the trench. The bladder is then deflated allowing the pipe to drop down to the unpadded trench floor while, as the pipe is being lowered, the unstable trench walls would collapse inward on top of the pipe. All three patents teach that the pipe is placed on the filled bag or bags, and then the bag is deflated, allowing padding or the bottom of the trench material to support the pipe. Each bag has a valve that is opened to allow deflation. Such air filled and water filled bags are more expensive than sandbags and foam blocks. Consequently, they are seldom used.
To insure that the pipe support benches perform the function of supporting the pipe above the trench floor for operating pipelines, it will be apparent to one of skill in the art that the benches must be constructed to be strong enough and be spaced close enough together so that the benches can adequately support more than the weight of the actual pipe itself. After the support benches are placed along the trench floor they will be subjected to the accumulative loads of the pipe itself, the fill material and the contents of the pipe.
FIG. 1 shows a typical pipeline installation of the prior art before backfilling. The pipeline 12 is supported on spaced apart benches 13 placed on the floor of a trench 14. For illustrative purposes, consider a steel pipe 36 inches in outer diameter with a wall thickness of 0.500 inches and with support benches located at 15 foot intervals along the trench floor, as shown in FIG. 1. When the pipe is laid on top of the spaced support benches, each support bench will experience a load of 2,860 pounds, assuming each bench will support an equal weight. As the trench is backfilled, the support benches will experience additional loading above the weight of the pipe alone. For the purposes of this example, we can estimate the additional loading during backfill to approximate xc2xd of the weight of the pipe. Employing this approximation, each bench will now experience a load of 4,290 pounds, again assuming each bench will support an equal weight. After the trench has been backfilled, the pipe is then filled with water and hydrostatically tested. In this example and again assuming equal bench weight distribution, each support bench will experience an additional load of 6,250 pounds for a total weight of approximately 10,540 pounds.
In the above example, dynamic loading has been ignored and it was assumed that each bench will experience equal loading. In the actual practice of installing pipelines, however, dynamic loading and unequal bench loading can exist. For example, if the bottom surface of a section of pipe does not align with the surface of the trench floor, it may be possible for individual support benches to experience increased loading several times as great as those shown in the above example. In FIG. 2, the bottom surface of the pipe 12 is shown to be out of alignment with the trench floor. As a result, support benches 1 and 4 are supporting the entire weight of the pipe section and the pipe is suspended above support benches 2 and 3. Support benches 1 and 4 are therefore supporting approximately twice the weight that would otherwise be supported had non-alignment not occurred. Pipeline installers are supposed to carefully observe pipeline installation to assure that the pipeline is supported by all of the benches. If gaps are seen like those in FIG. 2 the installer is required to lift the pipeline and insert one or more shims 15 as shown in FIG. 3. However, it is quite common for the installer to miss or even ignore gaps between benches and the pipeline especially when using expanded polystyrene benches.
With the necessity of insuring that the bottom surface of the pipe does not come into contact with the trench floor, and the risks of expensive pipe damage if it does contact the trench floor, the current method is to provide support benches constructed strong enough to support the entire expected loading calculated as in the above example plus an appropriate safety factor multiplier. Therefore, each bench will support the total weight or loading, namely, the weight of the empty pipe, the weight of the backfill, the weight of the water filled pipe during hydrostatic testing, the dynamic loading, and the increased weight caused by unequal bench loading. However, constructing the support benches rigid enough and placing the benches close enough together to support the total weight or loading as practiced in the current art makes it very likely that the position of the pipe after it has been initially lowered into the trench and placed on the support benches will remain fairly constant. That is, the height of the bottom surface of the pipe above the trench floor when the pipe is initially laid on top of the support benches will remain the same (or very nearly so) throughout the pipeline installation and testing processes.
As previously described, immediately after the pipe has been placed on top of the support benches, rock-free subsoil is introduced into the trench so that it flows around and under the pipe into the open area beneath the pipe between the support benches. Because the support benches have been constructed rigid enough to support the additional loads experienced after the placement of the padding material, the pipe cannot move to a position sufficiently low enough to compact the padding material beneath the pipe. Because the padding material beneath the pipe is not compacted, it will be readily affected by water, thus contributing to the well known problem of padding wash-out. Furthermore, when the padding material is not compacted, the pipe has no additional support after installation. Consequently, the installed pipe is supported by only the support benches themselves. In normal practice, it is not unusual that a typical underground pipeline be actually supported along less than 10 percent of its length. Unequal support bench loading can cause individual or a series of individual benches to be subjected to tremendous loading. Sometimes unequal loading has occurred in the practice of installing underground pipelines to such a degree that individual, or a series of individual benches fail, thereby allowing that particular section of pipe to drop to the trench floor.
Size and costs are other serious concerns with having the support benches constructed to support the total weight or loading on the pipe. It is necessary that these individual benches have a large enough surface area to prevent the pipe from being subjected to resultant point loading sufficient enough to cause the pipe to be flattened or ovalized (out-of-round). Should that occur an expensive dig-up and repair would be necessary.
Consequently, there is a need for a method of installing pipeline that permits installation without resultant uneven loading of support benches and yet is no more expensive or complicated than current installation methods.
I provide a method of installing pipeline using benches strong enough to support the pipe after the pipe has been lowered into the trench and padded. However, as opposed to the practice of the current state of the art, the benches are constructed to fail when additional cumulative loading produced by complete backfill and hydrostatic testing is applied. This method will accomplish beneficial outcomes not seen in the current art. A pipeline installed according to the present invention will be supported by the padding material beneath the pipe between the benches. As a result, a much greater percentage of the actual pipe will be supported. Further, the padding material beneath the pipe will be compacted, thereby greatly reducing the well-known problem of padding wash-out.